1. Field of the Invention
The present invention relates to rotary electric apparatus which are rotatable with precision and with minimized vibration and which are suitable as electric motors or generators, and more particularly to a rotary electric apparatus which comprises a field magnet and an armature with armature windings provided in the slots of its core and which is useful, for example, as a brushless motor for driving the head rotating cylinder of a video tape recorder.
2. Description of the Prior Art
With a conventional rotary electric apparatus which comprises, for example, an eight-pole field magnet and three-phase armature windings wound around 12 slotted portions of an armature core, 24 cogging phenomena occur per rotation of the armature relative to the field magnet.
FIG. 11 is a schematic front view showing the arrangement of the field magnet and the armature core of such a rotary electric apparatus which is a brushless electric motor. With reference to this drawing, the field magnet 1 serving as a rotor has eight poles, and the N poles are indicated by arrows toward the center, and the S poles by arrows in the centrifugal direction. The core 3 of the armature 2 serving as a stator has 12 slots 4 arranged at an equal spacing. Although not shown, the three-phase armature windings include four divided windings for each phase, with each divided winding wound around one salient pole. The four divided windings of each phase are wound in the same direction around the poles at a pitch of three slots.
In the illustrated case, the poles of the magnet 1 of the same polarity, e.g. N poles N1 to N4, are opposed to different slots 4 during rotation at the same time, so that the cogging forces combine to four times the single force. FIG. 12 shows variations in the cogging forces of the poles of the field magnet and variations in the combined cogging force thereof during 1/3 of a revolution.
As will be apparent from the diagram, the poles of the same polarity of the magnet 1 undergo cogging at the same angle of rotation, and the combined cogging force is four times the cogging force of each pole and is therefore a great value.
It appears possible to reduce the cogging force by diminishing the magnetic force of the magnetic poles, or by increasing the pole-to-pole distance between the field magnet 1 and the armature 2, but reduced efficiency and output will then result. Such prior art is disclosed, for example, in Examined Japanese Patent Publication No. SHO 49-8568.